The present invention relates to improvements in radio data communication systems wherein a number of mobile transceiver units are to transmit data to a base station under a wide range of operating conditions. The invention is preferably to be applicable as an upgrade of an existing data capture system wherein a number of hand held transceiver units of an earlier design are already in the field representing a substantial economic investment in comparison to the cost of a base station.
In the design of low-cost narrowband FM receivers, superheterodyne configurations using one or more fixed intermediate frequencies (IF) are generally employed. Receiver selectivity is provided using crystal or ceramic filters at these IF's. The passband characteristics of these filters must be wide enough to accommodate the bandwidth occupied by the intended received signal, plus any expected center frequency error due to frequency offsets in the transmitted signal or receiver local oscillators which may arise from changes in ambient temperature or misalignment. The stopband performance of the IF filters is chosen to provide the maximum out-of-band attenuation that is compatible with target design specifications, passband requirements, and equipment cost goals.
The technical requirements for narrowband FM equipment were originally devised for analog transmission of voice messages. Because the frequency spectrum of voice was easily quantifiable, it was possible to bandlimit the voice signal at baseband with no loss of information content, and utilize an optimal transmission bandwidth for acceptable signal quality at the receiver. Channel spacings were then derived based upon this optimal bandwidth, and the stability of the transmitter and receiver oscillator frequencies that could economically be achieved in portable and mobile equipment.
In UHF land-mobile equipment meeting the required transmit and receiver oscillator stabilities, the worst case frequency error is often comparable to the bandwidth of a transmitted voice signal. In order to accommodate some fraction of the worst case frequency error, it has been a common practice to substantially broaden the IF filter passband-widths.
The transmission of binary data through modified narrowband, FM, voice quality equipment requires special considerations. Unlike voice transmission, bandlimiting the signal limits the information transmission capabilities of the system. In order to obtain maximum information throughput, it is desirable to utilize the available bandwidth as fully as possible. However, for equipment to operate over a wide temperature range, transmitted bandwidth, and therefore data rate, must be constrained to keep the received signal within the IF filter bandwidth(s) when large frequency offsets due to oscillator temperature instabilities are present.
Unfortunately, when equipment is designed to tolerate large frequency offsets, a lower data rate must be selected than that allowed by the IF filter bandwidths under less stringent conditions where oscillator frequencies are near their ideal values.
Another consideration in data rate selection is system range. This is most problematic in data communications systems employing mobile or portable remote devices. These devices may operate at close range, where received signals are strong, or in fringe areas where received signals are extremely weak. Selecting a relatively high data rate will result in higher data throughput under strong signal conditions, while under fringe conditions, operation at lower data rates, with optimized baseband filtering, can be substantially more reliable.